The field of this invention is the measurement of nuclear magnetic resonance (NMR) for the purpose of determining molecular or microscopic structure, and, more particularly, a multi-resonant circuit for NMR measurements requiring simultaneous high-power excitation of a plurality of nuclides.
The NMR spectroscopist often finds it necessary to observe a Wide variety of nuclides, especially .sup.13 C, .sup.1 H, .sup.19 F, .sup.27 Al, .sup.29 Si, .sup.2 H, and .sup.15 N in the study of commercially and scientifically important chemicals. It is often desirable for the NMR circuit to be able to tune to two or more frequencies simultaneously. The most common example is irradiating at the proton (.sup.1 H) high frequency (HF) resonance to decouple its dipolar broadening effects while observing Bloch decays on a nuclide of lower magnetogyric ratio such as .sup.13 C at a low frequency (LF). Other examples include cross-polarization and inverse detection.
For-experiments on solid samples at high static magnetic field B.sub.o (greater than 6 T), where large RF fields B.sub.1 (greater than 0.6 mT) are required, typical voltages across the sample coil (also called the excitation coil) are 2-6 kV. In U.S. Pat. No. 4,710,719, I disclose an efficient method of achieving broadband tuning of a single-tuned circuit or of the lower frequency channel of the standard double-tuned, high-power circuit for narrow-bore magnets. In "Noise in High Power, High Frequency Double Tuned Probes," J. Mag. Res. Vol. 77, p. 536 (1988), by F. D. Doty, T. J. Connick, X. Z. Ni, and M. N. Clingan, the authors discuss the various requirements for a high-performance, high-field, double-tuned solids probe for NMR with the standard MAS circuit as depicted in FIG. 1. This circuit has been found to have a practical (marginally acceptable reliability) upper limit proton B.sub.1 of about 1.4 mT at 400 MHz for a 7 mm sample spinner diameter--or 1.2 mT for high reliability. More information on typical spinner systems may be found in copending application 07/607,521 filed Nov. 1, 1990 and in references cited therein.
In U.S. Pat. No. 4,833,412, Zens discloses a method of double tuning a balanced saddle coil for fixed-frequency high resolution experiments. The proton B.sub.1 reported therein is about 0.23 mT (26 .mu.s 90.degree. pulse length), although it appears likely that his approach could be extended to 0.5 mT on small volumes. However, the enormous stray capacitance associated with his quarter-wavelength technique imposes severe limits on broadband tuning. (A balanced coil is one which is virtually grounded at its center.)
In U.S. Pat. No. 4,742,304, Schnall et al. disclose a method of double tuning that does not achieve balance and results in poor LF efficiency when sample losses are small--as is the case for most solids NMR experiments. It too affords limited broadband tuning capability. The same is true of the technique disclosed by McKay in U.S. Pat. No. 4,446,431.
In some cases (e.g., high density polymers) it is desirable to achieve 2 mT proton B.sub.1 at 400 MHz with high reliability in double-resonance circuits. This requires smaller sample spinners (4-5 mm) with prior art circuits, and hence gives lower sensitivity. In other cases, it would be desirable to achieve 1.2 mT, 300 MHz proton B.sub.1 with 14 mm sample spinners, In all double-tuned NMR circuits, it is desirable to reduce the shot noise associated with ground loops through high resistance contacts in the probe structure and to minimize other sources of signal degradation, including low filling factor, low efficiency, and low Q. It is also often desirable to obtain broadband operation, variable temperature operation, and high-speed sample spinning. The above combined performance objectives have been well beyond the capability of available circuits because of high voltage breakdown, limited tuning range from parasitic capacitance and inductance, or poor efficiency. The instant invention satisfies the above objectives.
It is well known that an electrically balanced circuit results in reduced radiation, lower voltages with respect to ground, and reduced sample losses. The double-tuned circuit shown in FIG. 2 has been widely used in fixed-frequency, low-power (typically under 300 V across the coils), high-resolution probes for liquid NMR samples for nearly two decades. It may be balanced at the HF by proper tuning. (Strictly speaking, it is a triple-resonance circuit, as it has three distinguishable inductors and more than three distinguishable capacitors. However, only two modes, the lowest and the highest distinguishable, result in substantial current through the central coil L1 when the 10 circuit is balanced. Hence, it is commonly referred to as a double-resonance circuit.) The instant invention discloses an improvement of this circuit such that HF balance is maintained while permitting (1) high-power operation (typically 2-6 kV), (2) broadband tuning of the LF mode, and (3) high efficiency at both (useful) frequencies.